The present invention relates to a nuclear power plant and a method of reducing the radioactivity in the power plant and, more particularly, to a nuclear power plant in which iron crud in the primary cooling water are fixed and which is reduced in the radioactivity.
In nuclear power plants, the quality control of a primary cooling water is effected to reduce radiation dose which the workers working there receive. Corrosion products (iron crud), which are produced from pipings of a primary cooling water line and comprise iron oxides and iron hydroxides as main components, flow into a reactor together with nickel and cobalt which are convertible to radionuclides Co.sup.58 and Co.sup.60 by radiation, respectively. In the reactor, the iron crud, nickel and cobalt are adhered to the surfaces of fuel claddings and exposed to neutron radiation there. As a result, the nickel and the cobalt are converted into Co.sup.58 and Co.sup.60, respectively. The radioactivated nuclides such as Co58, Co.sup.60, etc. dissolve from the cladding surfaces into the reactor water, and adhere to, precipitate in pipings in the reactor, whereby radiation dose in a reactor water recirculation line, a condensed water line, a feed water line, etc. increases, which causes an increase in radiation exposure to the workers. The radiation dose is considered possible to be suppressed by reduce an amount of the iron crud. Therefore, various measures for reducing the iron crud have been taken, for example, they are an improvement on the condensed water purification system line, oxygen injection into the feed water line, use of anticorrosive material, etc.
However, when iron crud concentration in the feed water becomes very low, a balance between the iron crud and nickel, cobalt adhering to the fuel rod cladding surfaces, is broken, and an amount of nickel, cobalt relative to the iron becomes large. Therefore, dissolution speed of Co.sup.58, Co.sup.60 from the fuel rod cladding surface increases and radioactivity in the reactor water is raised, so that the radiation dose in the reactor pipings is raised. In order to reduce the radiation dose, it is necessary to reduce the dissolution speed of Co.sup.58, Co.sup.60 from the fuel rod cladding surface. Therefore, it is necessary to convert the nickel, cobalt into nickel ferrite, cobalt ferrite which are compound oxides of nickel, cobalt and iron and low in dissolution speed.
In order to form the nickel ferrite or cobalt ferrite, stoichiometrically, iron of 2 mols of is needed to 1 mol of nickel, cobalt of 1 mol, however, a conversion rate to the ferrite does not become 100%, so that the iron is needed more than the stoichiometric amount. As mentioned above, in order to reduce radiation dose of Co.sup.58, Co.sup.60 from the piping surfaces, it is effective to convert Co.sup.58, Co.sup.60 to nickel ferrite, cobalt ferrite. In a BWR power plant, control of iron concentration to nickel and cobalt (in particular to the nickel which is more than the cobalt) has been effected. As a method of controlling the iron concentration, there is a method of controlling an amount of iron crud flowing in the reactor by causing part of condensed water to by-pass a condensed water filter which is one of two kinds of condensed water purification apparatus. Further, there is a method of injecting iron hydroxides, iron oxides and iron ions into a cooling water in pipings between a downstream side of a condensed water purification apparatus and the reactor, as described in Japanese Patent Laid-Open Nos. 61-240196, 61-245093.
In the above-mentioned prior arts, a conversion rate of nickel and/or cobalt into nickel ferrite and/or cobalt ferrite through reaction between the nickel and/or cobalt and iron component such as in iron oxide, iron hydroxide, iron ion is low and a large amount of the iron crud is produced, so that the radioactivity can not be reduced sufficiently.